1. Field of the Invention
The present invention relates to mobile radio telephone systems and methods for operating mobile radio telephone systems.
2. Description of the Related Art
Mobile radio telephone systems serve to provide mobile units with a means for communicating data via radio signals while the mobile units move within the mobile radio system. To this end, mobile radio systems are provided, inter alia, with a plurality of interconnected base stations which form a mobile radio network.
In order to make optimum use of a portion of radio frequency spectrum allocated to a mobile radio system, the mobile radio system is provided with means to maximize a number of data communications channels which may be contemporaneously supported within a given radio coverage area. The data communications channels may, for example, carry data representative of speech signals forming part of a telephone conversation. Data is communicated between the mobile and the base station by modulating radio signals formed from electromagnetic waves having a high frequency which serve to carry the data between mobile stations and the base stations. The radio signals modulated by the data are known as carrier signals, and the frequency of the carrier signals is known as the carrier frequency. Each carrier signal has a radio frequency bandwidth centered around the carrier frequency, which therefore occupies part of the radio frequency spectrum allocated to the mobile telephone system. A portion of the radio frequency spectrum allocated to the mobile radio telephone system is therefore occupied by a plurality of carrier signals, each carrier signal forming a communications channel.
In order to make optimum use, therefore, of the allocated radio frequency spectrum, mobile radiotelephone systems are arranged to re-use communications channels formed by the radio frequency carrier signals by assigning the same carrier frequencies to different base stations within the system, and thereby re-using the carrier frequencies. Although this results in co-channel interference being generated at each base station, this interference may be reduced to a tolerable level by arranging for base stations using the same carrier frequencies to be separated from each other by as large a distance as possible. This is facilitated by a reduction in signal strength in proportion to the distance traveled by the radio signals, in accordance with the natural laws of propagation.
The term co-channel interference as used herein refers to an amount of unwanted signal power generated by a mobile or a base station within the mobile radio system on the same reallocated carrier frequency at a receiver operating to detect wanted signals at another base station or mobile station. Co-channel interference has the effect of reducing the integrity of the communicated data.
An example of a mobile radio system is the Global System for Mobiles (GSM) which has become widely adopted throughout the world. For example, in Europe, over 20 million people are known to use the GSM system or a similar derivative of the GSM system which is known as DCS 1800. In order to effect a frequency re-use, operators of the GSM system deploy base stations in order to effect a radio coverage over a certain geographical location and allocate to those base stations carrier frequencies in accordance with a frequency re-use pattern.
A frequency re-use pattern is known to those skilled in the art as being an arrangement wherein a plurality of carrier frequencies are allocated to a plurality of base stations in accordance with a predetermined pattern which pattern is thereafter repeated for other base stations within the system.
A geographical area associated with a base station within which a mobile station can effect communication of data via radio signals with the base station, in preference to another base station is known as a cell. The frequency re-use pattern may therefore be viewed as forming a plurality of cell clusters, each cluster having the same set of carrier frequencies, but each call or each sector of the cell in the cluster being allocated different carrier frequencies.
A sectored cell is formed using directional antennas which serve to further divide a cell into a plurality of parts known as sectors, to which different carrier frequencies may be assigned. Therefore, a mapping of the frequency re-use pattern of the mobile radio system onto cells or cell sectors will result in an associated cluster, the size of the cluster corresponding to the number of separate sectors or cells onto which the set of carrier frequencies re-used in accordance with the frequency re-use pattern is mapped.
The term cluster size as used herein therefore refers to and includes a number of sectors or cells onto which the set of carrier frequencies formed are mapped in accordance with the frequency re-use pattern.
As will be appreciated, the cluster size is determined in accordance with the number of radio frequency carriers available to the mobile radio telephone system in combination with an amount of co-channel interference which can be tolerated by the radio communications apparatus, while maintaining an acceptable integrity of the communicated data. The capacity of the mobile radio communications system, in terms of the number of data communications channels which can be contemporaneously supported within a geographical area and within a given portion of radio frequency spectrum, can therefore be seen to increase, both as the size of the cell cluster reduces and as the geographical area associated with a cell is reduced. For example, operators of the GSM system are known to use a cluster size of nine.
The GSM system is a second generation mobile radio system which employs a time division multiple access technique, which serves to support a plurality of data channels contemporaneously. However, it is known that third generation mobile radio systems will supercede the second generation mobile radio systems and provide advantages associated with an increase in variety of data types which may be communicated over the mobile radio system, and which will provide an increase in capacity which the system can support.
Considerable investment has been made by operators in procuring and deploying second generation mobile radio systems, and in particular, the operators of the GSM, DCS 1800 and GSM 1900 systems. Providing a mobile radio system with a means for combining different radio communications apparatus, within an allocated radio frequency spectrum represents a technical problem which is addressed by the present invention.